Bistable LCD Switches Optically

With the popularity of portable electronic devices that incorporate LCDs, researchers have been exploring ways of reducing power consumption to enable those cell phones, laptop computers and personal digital assistants to run longer on a single battery charge. Of recent interest is the bistable display, in which a voltage switches the liquid crystal between stable states with different optical properties and thus need not be applied continuously to produce a desired image, as in traditional monostable LCDs.

Bistable displays, whether incorporating electrophoretic particles or cholesteric liquid crystal, have been demonstrated for applications such as “electronic paper,” in which the flexible devices generate one “page” of text and graphics and do not consume power again until the next page is produced.

In proof-of-principle experiments, scientists at National Cheng Kung University in Tainan, Taiwan, have shown that the liquid crystal in a bistable LCD need not be electrically driven. Instead, they used different polarizations of laser light to switch E7 liquid crystal doped with methyl red dye at 1 percent by weight between states.

The bistable LCD is switched optically between dark “off” and light “on” states. When exposed to linearly polarized laser light, dye molecules in the device reorient the liquid crystal to create a 45° twisted nematic phase. Illumination with circularly polarized laser light returns the material to its initial state. Reprinted with permission of Applied Physics Letters.They sandwiched a mixture of the liquid crystal and dye between ITO-coated glass slides spaced 6 μm apart — one coated with a polyimide and rubbed to promote the homogeneous alignment of the liquid crystal parallel to the plane of the slide, and the other coated with a UV-cured polymer. On the exterior of this cell, they attached a polarizer to the side treated with the polyimide and a λ/4 plate and partially transmissive mirror to the side coated with the polymer.

When illuminated on the polymer side with linearly polarized 532-nm light, the dye molecules undergo isomerization and are adsorbed on the polymer layer such that their long axes are oriented vertically to the polarization of the laser beam, causing the liquid crystal there to be oriented similarly. On the other side of the cell, however, the liquid crystal remains anchored by the rubbed polyimide. As a result, a 45° twisted nematic phase is created, and the LCD achieves the stable “on” state at the point of illumination. To return the material to the stable “off” state, it is illuminated with circularly polarized light.

The researchers employed a diode-pumped solid-state laser with an intensity of 150 mW/cm2 in the experiments, and the switching time was ~1 min. Nevertheless, they suggest that the method is promising for display applications.